The use of zeolite adsorbents comprising at least one faujasite (FAU) zeolite of type X or Y and comprising, besides sodium cations, barium, potassium or strontium ions, alone or as mixtures, for selectively adsorbing para-xylene in a mixture of aromatic hydrocarbons, is well known in the prior art.
U.S. Pat. Nos. 3,558,730, 3,558,732, 3,626,020 and 3,663,638 show that zeolite adsorbents comprising aluminosilicates based on sodium and barium (U.S. Pat. No. 3,960,774) or based on sodium, barium and potassium, are effective for separating para-xylene present in aromatic C8 fractions (fractions comprising aromatic hydrocarbons containing 8 carbon atoms).
The adsorbents described in U.S. Pat. No. 3,878,127 are used as adsorbing agents in liquid-phase processes, preferably of simulated counter-current type, similar to those described in U.S. Pat. No. 2,985,589 and which apply, inter alia, to aromatic C8 fractions.
In the patents listed above, the zeolite adsorbents are in the form of crystals in powder form or in the form of agglomerates consisting predominantly of zeolite powder and up to 20% by weight of inert binder.
The synthesis of FAU zeolites is usually performed by nucleation and crystallization of aluminosilicate gels. This synthesis leads to crystals (generally in powder form) whose use at the industrial scale is particularly difficult (substantial pressure losses during handling). Agglomerated forms of these crystals are thus preferred, in the form of grains, yarns and other agglomerates, these said forms possibly being obtained by extrusion, pelletizing, atomization and other agglomeration techniques known to those skilled in the art. These agglomerates do not have the drawbacks inherent in pulverulent materials.
Moreover, zeolite crystals are usually prepared from aqueous sodium solutions (for example aqueous sodium hydroxide solution), and, if so desired, the sodium cations may be replaced (exchanged) totally or partly with other cations, for example barium or barium and potassium. These cationic exchanges may be performed before and/or after agglomeration of the pulverulent zeolite with the agglomeration binder, according to standard techniques known to those skilled in the art.
The agglomerates, whether they are in the form of platelets, beads, extrudates or the like, generally consist of zeolite crystals, which constitute the active element (in terms of adsorption) and an agglomeration binder. This agglomeration binder is intended to ensure the cohesion of the crystals to each other in the agglomerated structure, but also should make it possible to ensure sufficient mechanical strength for said agglomerates so as to avoid, or at the very least minimize the risks of fractures, cracks or breaks that might arise during their industrial uses during which the agglomerates are subjected to numerous constraints, such as vibrations, large and/or frequent variations in pressure, movements and the like.
The preparation of these agglomerates is performed, for example, by slurrying zeolite crystals in powder form with a clay paste, in proportions from about 80% to 90% by weight of zeolite powder per 20% to 10% by weight of binder, followed by forming into beads, platelets or extrudates, and heat treatment at high temperature to bake the clay and reactivate the zeolite, the cationic exchange(s), for instance the exchange with barium and optionally with potassium, possibly taking place before and/or after agglomeration of the pulverulent zeolite with the binder.
Zeolite substances whose particle size is a few millimeters, or even of the order of a millimeter, are obtained, and which, if the choice of the agglomeration binder and the granulation are made within the rules of the art, have a satisfactory set of properties, in particular of porosity, mechanical strength and abrasion resistance. However, the adsorption properties of these agglomerates are obviously reduced when compared with the starting active powder due to the presence of agglomeration binder which is inert with respect to adsorption.
Various means have already been proposed for overcoming this drawback of the agglomeration binder being inert towards adsorption performance, among which is the transformation of all or at least part of the agglomeration binder into zeolite that is active from the point of view of adsorption. This operation is now well known to those skilled in the art, for example under the name “zeolitization”. In order readily to perform this operation, zeolitizable binders are used, usually belonging to the kaolinite family, and preferably calcined beforehand at temperatures generally between 500° C. and 700° C.
Patent application FR 2 789 914 describes a process for manufacturing zeolite X agglomerates, with an Si/Al atomic ratio of between 1.15 and 1.5, exchanged with barium, and optionally with potassium, by agglomerating zeolite X crystals with a binder, a source of silica and carboxymethylcellulose, followed by zeolitizing the binder by immersing the agglomerate in an alkaline liquor. After exchange of the cations of the zeolite with barium ions (and optionally potassium ions) and activation, the agglomerates thus obtained have, from the point of view of adsorption of para-xylene contained in aromatic C8 fractions, improved properties when compared with adsorbents prepared from the same amount of zeolite X and of binder, but whose binder is not zeolitized.
U.S. Pat. No. 7,812,208 (UOP) describes a process for separating para-xylene contained in aromatic fractions, using an adsorbent of “binderless” type, i.e. without amorphous material or with an amount of less than 2% by weight of amorphous material, based on zeolite X, with a mean crystal size of less than 1.8 μm. These adsorbents are obtained after a step of zeolitization of the binder.
These adsorbents have improved transfer and adsorption properties and do not contain, or only in an amount of less than 2% by weight, and usually less than 0.5% by weight, amorphous or non-zeolitic material. On the other hand, no information is given regarding the mechanical strength of such “binderless” particles. Said document teaches that a total conversion of the binder into zeolite would make it possible to maximize the adsorption capacity. However, the mechanical properties do not always appear to be conserved or optimized in this case.
This is confirmed, for example, by patent application FR 2 999 098, which describes an agglomerated zeolite adsorbent based on zeolite X with small crystals typically less than 1.7 μm in size and which has maximum selectivity properties towards para-xylene and matter transfer properties. For this type of adsorbent, a compromise is imposed between maximum mechanical strength and optimized adsorption capacity. It also emerges in the light of the examples that even after optimum zeolitization, the smaller the size of the starting zeolite crystals (for example 0.8 μm), the weaker the mechanical strength of the agglomerated adsorbents.
The preparation processes described in the prior art involve an additional zeolitization step which, besides potentially degrading the crystallinity of small-sized crystals (<0.5 μm), entails additional costs.
Besides a high adsorption capacity and good selectivity properties towards the species to be separated from the reaction mixture, the adsorbent must have good matter transfer properties so as to ensure a sufficient number of theoretical plates for achieving efficient separation of the species in mixture, as indicated by Ruthven in the book entitled Principles of Adsorption and Adsorption Processes, John Wiley & Sons, (1984), pages 326 and 407. Ruthven indicates (ibid., page 243) that, in the case of an agglomerated adsorbent, the overall matter transfer depends on the addition of the intra-crystalline diffusional resistance and the diffusional resistance between the crystals.
The intra-crystalline diffusional resistance is proportional to the square of the diameters of the crystals and inversely proportional to the intracrystalline diffusivity of the molecules to be separated.
The diffusional resistance between the crystals (also known as the “micropore resistance”) is itself proportional to the square of the diameters of the agglomerates, inversely proportional to the porosity contained in the macropores and mesopores (i.e. the pores whose aperture is greater than 2 nm) within the agglomerate, and inversely proportional to the diffusivity of the molecules to be separated in this porosity.
The size of the agglomerates is an important parameter during the use of the adsorbent in industrial application, since it determines the pressure loss within the industrial unit and the packing uniformity. The particle size distribution of the agglomerates should thus be narrow, and centred on number-average diameters typically between 0.40 mm and 0.65 mm so as to avoid excessive pressure losses.
The porosity contained in the macropores and mesopores may be increased by using pore-forming agents, for instance corn starch as recommended in document U.S. Pat. No. 8,283,274 for improving the matter transfer. However, this porosity does not participate in the adsorption capacity and, consequently, the improvement in the macropore matter transfer then takes place to the detriment of the volume adsorption capacity. Consequently, this approach for improving the macropore matter transfer proves to be very limited.
To estimate the improvement in the transfer kinetics, it is possible to use the plate theory described by Ruthven in Principles of Adsorption and Adsorption Processes, ibid., pages 248-250. This approach is based on the representation of a column by a finite number of ideally stirred hypothetical reactors (theoretical stages). The equivalent height of theoretical plates is a direct measurement of the axial dispersion and of the resistance to matter transfer of the system.
For a given zeolite structure, a given size of adsorbent and a given operating temperature, the diffusivities are fixed, and one of the means for improving the matter transfer consists in reducing the diameter of the crystals. A gain on the overall matter transfer will thus be obtained by reducing the size of the crystals.
A person skilled in the art will thus seek to minimize the diameter of the zeolite crystals in order to improve the matter transfer.
Patent CN 1267185C thus claims adsorbents containing 90% to 95% of zeolite BaX or BaKX for the separation of para-xylene, in which the zeolite X crystals are between 0.1 μm and 0.4 μm in size, in order to improve the matter transfer performance. Similarly, patent application US 2009/0 326 308 describes a process for separating xylene isomers in which the performance was improved by using adsorbents based on zeolite X crystals with a size of less than 0.5 μm.
The Applicant has nevertheless observed that the synthesis, filtration, handling and agglomeration of zeolite crystals whose size is less than 0.5 μm involve cumbersome, uneconomical processes that are thus difficult to industrialize.
Furthermore, such adsorbents comprising crystals less than 0.5 μm in size also prove to be more fragile, and it then becomes necessary to increase the content of agglomeration binder in order to reinforce the cohesion of the crystals within the adsorbent. However, increasing the content of agglomeration binder leads to densification of the adsorbents, which is the cause of an increase in the macropore diffusional resistance. Thus, despite a reduced intra-crystalline diffusional resistance due to the decrease in the size of the crystals, the increase in macropore diffusional resistance on account of the densification of the adsorbent does not allow an improvement in the overall transfer.
Moreover, increasing the binder content does not makes it possible to obtain good adsorption capacity.
The final adsorption capacity may be improved by performing, as taught in the prior art, zeolitization of the agglomeration binder of the adsorbent.
However, the beneficial effect of this binder conversion step may be greatly penalized by the degradation in crystallinity of the starting nanocrystals, this degradation being caused by the basic solutions used during this zeolitization step.
A third property of the adsorbent that is necessary for ensuring good performance of the liquid-phase separation process of simulated counter-current type is to have good mechanical strength. Specifically, under standard operating conditions of this type of process, a high mechanical stress is applied to the adsorbent in the industrial units, entailing the formation of fine particles, which induce a deterioration in the performance (see, for example, Primary Analysis on State of Xylene Adsorption Unit, Li et al., Jingxi Shiyou Huagong, 2004, (4), 54-55), and this being all the more the case the lower the mechanical strength of the adsorbent.
However, the prior art FR 2 999 098 shows that when small-sized crystals (for example 0.8 μm) are used, the mechanical strength also reduces, despite the zeolitization step. A person skilled in the art would thus tend to increase the size of the crystals in order to improve the mechanical strength.
In summary, for the separation of xylenes, the prior art shows that it is necessary:                1) to reduce the size of the crystals in order to improve the matter transfer,        2) and/or to increase the macroporosity by using pore-forming agents, and        3) to zeolitize the binder in order to increase the mechanical strength and maximize the adsorption capacity.        
It thus appears difficult to obtain adsorbents having all the following properties combined:                the fastest possible matter transfer within the adsorbent, i.e. the smallest possible and ideally virtually zero, or even zero, resistance to matter transfer,        optimum mechanical crushing strength,        the greatest possible adsorption capacity (i.e. a content of zeolite (active crystalline phase for the purposes of adsorption) that is as large as possible).        